Transform coding is an efficient image compression scheme that typically involves segmenting an image into blocks of pixels, taking discrete cosine transforms ("DCTs") of the blocks of pixels to obtain blocks of DCT coefficients, quantizing these coefficients, and coding the quantized coefficients by an entropy coder. Interframe coding schemes utilizing motion compensation and transform coding of motion compensated interframe differences, by taking DCTs of blocks of difference pixels, quantizing the DCT coefficients and entropy coding the quantized DCT coefficients, may also be employed.
Interframe coding employing motion compensation and DCT coding has become widely recognized as a particularly efficient coding scheme for video compression and forms the core of the Comite Consulatif International Telegraphique et Telephonique Recommendation H.261-Video Codec for Audiovisual Services at 64 Kbit/s, Geneva, August, 1990 ("CCITT H.261") and the Motion Pictures Expert Group Phase 1 ("MPEG-1") video compression standards. The MPEG-1 standard is set forth in International Standards Organization ("ISO") Committee Draft 11172-2, "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to 1.5 Mbits/s," November, 1991. The CCITT H.261 standard primarily addresses coding of video conferencing scenes of Common Intermediate Format resolution at bitrates of 64 kbit/s to 2 Mbit/s; the MPEG-1 standard can be efficiently used for coding all types of video scenes in 1 to 2 Mbit/s range. The MPEG-1 standard incorporates additional features, for example, group-of-pictures and motion compensated bidirectional prediction. However, because the MPEG-1 standard was initially applied to progressive format video, it is not optimized for coding of interlaced video conforming to the Comite Consultatif International des Radiocommunications Recommendation 601 ("CCIR 601") format which is similar to that used for broadcast and cable television. The CCITT H.261 and MPEG-1 coding standards specify that the DCT coefficients must be ordered before they are encoded. This ordering of DCT coefficients prior to encoding is called "scanning." For progressive format video, a conventional zigzag scan of DCT coefficients mentioned in the standards allows for ordering of the coefficients relative to their significance from low frequency to high frequency which results in events that can be efficiently coded by a two-dimensional variable length coder. Unfortunately, conventional zigzag scans do not allow for efficient coding of interlaced format video signals.
One approach to video coding using alternatives to the zigzag scan is discussed in U.S. Pat. No. 5,227,878 by A. Puri et al. which refers to adaptive coding on a macroblock basis using frame or field coding.